Yes, get on the Heritage Foundation mailing list and you'll never be lonely, although the conversation may get a little monotonous. But there is one puzzling thing. Almost every mailing from the Heritage Foundation says at the bottom of the first page, ''Note: Nothing written here is to be construed . . . as an attempt to aid or hinder the passage of any bill before Congress.''

There is actually no mystery. The Heritage Foundation is a tax-exempt institution, and contributions to it are tax-exempt. The law warns that ''carrying on propaganda, or otherwise attempting to influence legislation'' is verboten for tax-exempt institutions.

So it's good to know that the Heritage Foundation is spending $20 million a year not attempting to influence legislation. What is it attempting to do? The law limits tax-exempt institutions to certain high-minded purposes. Is the Heritage Foundation attempting ''to foster national or international amateur sports competition'' or ''the prevention of cruelty to children or animals?''

Perhaps its purpose is ''literary:'' to make surrealistic comment on the absurdity of modern life. A piece of paper that says at the top, ''The National Competitiveness Act (S.4): a High-Tech Boondoggle,'' and says at the bottom that it is not intended to aid or hinder legislation before Congress is rather like Magritte's famous painting of a pipe, labelled ''Ce n'est pas une pipe.''

Heritage is probably within the law as interpreted by IRS regulations. The rule seems to be that tax-exempts can say anything they wish about a piece of legislation, as long as they don't specifically urge a vote for or against it. But the question remains why anyone should believe anything else in a Heritage handout when there's an obvious whopper on the bottom of the first page.

Other Washington think tanks, which are also tax-exempt, don't print any such this-is-not-propaganda disclaimer on their propaganda. But then no other Washington think tank goes as far as Heritage in blurring the distinction between a genuine educational institution and an out-and-out propaganda machine.

On some of its mailings, the Heritage Foundation subtitles itself, ''A Tax-Exempt Public Policy Research Institution.'' But according to its most recent annual report, Heritage spends just 45 percent of its annual budget on research. Thirty-seven percent goes to ''marketing,'' 13 percent goes to fund-raising, .. and five percent goes to management. Somehow, though, ''A Tax-Exempt Public Policy Marketing and Fund-raising Institution'' doesn't have the right ring.

By contrast the other major conservative think tank, the American Enterprise Institute, spends just 10 percent of its budget on marketing and fund-raising combined.

When I first came to Washington in the 1970s, AEI was the cutting-edge right-wing agitprop organization. But it has long-since been surpassed by Heritage. This is partly because AEI developed ''Brookings Envy'' and lost some of its partisan edge. But it's partly because Heritage pushed the outer limits of what was acceptable partisanship in a so-called ''think tank.''

Institutions like Heritage give conservatives a major advantage in the public-policy debate. There is no liberal equivalent of the Heritage Foundation -- and, given the reality of raising money from corporations and wealthy individuals, there never will be.

There is, of course, the Brookings Institution. Brookings is still probably more prestigious than Heritage, and its product is probably, on balance, liberal. But Brookings is not ideologically committed in anything like the same way as Heritage. It is much more genuinely a scholarly institution, more concerned with research than advocacy. And anyway, Heritage's annual budget has now surpassed that of Brookings.

Conservatives, so concerned about the debasement of intellectual life in academia, have done their bit to debase the notion of scholarship themselves with faux-academic institutions that mimic the life of the mind -- down to paraphernalia like endowed chairs -- in the service of a considerably less disinterested ideal.

But give Heritage some credit. It is fighting the war of ideas, usually with honesty and always with the vigor of true belief. And without the Heritage Foundation, some days I wouldn't get any mail at all.